1. Technical Field
The present invention relates to protection circuits which protect battery packs including secondary batteries from overcurrent and abnormal overheating. More particularly, the invention relates to battery pack protection circuits including positive temperature coefficient thermistors and switching elements, and battery packs related thereto.
2. Background Art
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-152516, incorporated by reference.
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-8608, incorporated by reference.
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-120849, incorporated by reference.
In battery packs used as power supplies for mobile phones and the like, it has been required to provide two means of protection against overcurrent and overheating. Patent Document 1 shows a structure of a known protection circuit used for that purpose.
An example of a battery pack protection circuit disclosed in Patent Document 1 is shown in FIG. 11. In FIG. 11, a battery cell 131 is provided with a first switching element 134a and a second switching element 134b in the charge/discharge path and a protective control circuit 133 which controls them. A protective element 132, which in this example is a positive temperature coefficient thermistor, is connected in series in the charge/discharge path in the vicinity of the battery cell 131. If the switching elements 134a and 134b are abnormally overheated, resulting in thermal runaway or failure, even if the protective control circuit 133 detects the abnormality, since the switching elements 134a and 134b cannot be shut off, the protection function may not work. Even in such a case, because of the behavior of the protective element 132, it is possible to ensure the protection function against overcurrent and overheating of the battery cell.
Use of a polymer PTC element as an example of a positive temperature coefficient thermistor constituting a protective element 2 is described in Patent Document 2.
Furthermore, Patent Document 3 discloses a method in which, when a predetermined signal is applied to a first or second signal input terminal provided on a protective control circuit, a switching element is controlled to shut off the current path. In this method, when a signal showing that an abnormal current flows in the first signal input terminal, the switching element is controlled to shut off the current path. In addition, when a signal showing the temperature of the entire battery pack is input as the predetermined signal to the second signal input terminal, the current path is also shut off. That is, Patent Document 3 discloses a structure of a battery pack protection circuit in which an increase in the temperature of the entire battery pack is detected, and using the protection function of a protective control circuit, the current path is shut off when an abnormal temperature occurs.
The polymer PTC element used in Patent Document 2 as the protective element 2, however, has a heavy overall weight and a large volume and is of a lead type. Thus, when the polymer PTC element is incorporated into a battery pack, it is necessary to perform manual soldering and welding, resulting in an increase in manufacturing cost. Furthermore, even if a temperature fuse is used as the protective element 2, because of its heavy overall weight and large volume and because it again is of a lead type, the manufacturing cost is increased, as in the case of the polymer PTC element. Moreover, when the temperature fuse is disconnected due to an abnormal temperature, it cannot be reused. Furthermore, the protection function does not work against overcharge until the battery cell overheats and the protective element is activated. Thus, abnormal overheating of the switching elements 134a and 134b in itself cannot be protected against. Consequently, there is a possibility that the outer resin case of the battery pack may be deformed or melted due to abnormal overheating.
To overcome these problems, Patent Document 1 proposes a protection circuit in which a third switching element such as an FET is provided in place of the protective element 132, and the third switching element is controlled using a change in resistance with temperature of a negative temperature coefficient thermistor or a positive temperature coefficient thermistor provided in the battery pack. Thus, even when the protective control circuit 133 fails to function, it is possible to protect the battery pack from overcurrent during charging/discharging.
However, since the third switching element is inserted in series into the charge/discharge current path of the battery cell, an expensive power FET is required, and the cost is increased. Furthermore, use of the third switching element increases the possibility of thermal runaway or failure due to abnormal overheating.
On the other hand, in the battery pack protection circuit described in Patent Document 3, no resistor component is inserted in series into the current path. In the protection circuit according to Patent Document 3, the increase in temperature of the entire battery pack is detected, and when an abnormal temperature occurs, the switching element is controlled by the protective control circuit to shut off the current path. Consequently, it is not possible to detect a local increase in temperature in the battery pack. In reality, it is the switching element that is first brought into a high-temperature state by an overcurrent. Consequently, if only the switching element is in a high-temperature state, the entire battery pack may not yet reach a high-temperature state that allows the protection function to work. In such a case, the switching element is subjected to thermal runaway. Therefore, in some cases, the temperature of the entire battery pack is increased after the thermal runaway of the switching element, and even if it is attempted to control the switching element by the protective control circuit, the control may not be possible. That is, in the protection circuit described in Patent Document 3, since it is not possible to detect local overheating to protect the circuit, the protection function is not sufficient.
If this problem is addressed by setting a low protection-start temperature, there is a possibility that the protection operation may be activated even by overheating of a degree that causes no problem in reality.